Hollow body molding devices are known which form a hollow molded body in a main cavity of a mold having in its one end a pressure port including a floating core by injecting molten resin through an injection gate into the main cavity having an outlet in its other end, and then pressure-injecting pressurized fluid through the pressure port into the main cavity to move the floating core toward the outlet through the molten resin injected into the main cavity so as to extrude the molten resin through the outlet and discharge the extruded molten resin into a waste cavity. Such hollow body molding devices are described in, e.g., Patent Documents 1 and 2.